Dampening system for planographic presses



July 17, 1928. 7 1,677,655

R. F. REED ET AL DAMPENING SYSTEM FOR PLANOGRAPHIC PRESSES Filed Feb. 15, 1927 vweutou 35% h mJ M attoz wgo Patented July 17, 1928 UNITED STATES PATENT OFFICE.

ROBERT F. REED, OF NORWOOD, OHIO, AND WALTER SCHOLL, OF WASHINGTON, DIS- TRICT OF COLUMBIA, ASSIGNORS TO THE LITHOGRAPHIC TECHNICAL FOUNDA- TION, INC., 01 NEW YORK, N. Y., A CORPORATION OF DELAWARE.

DAMPENING SYSTEM FOR PLANOGRAPHIC PRESSES.

Application filed February 15, 1927. Serial No. 168,457.

This invention relates to the dampening system of planographic printing presses using metal printing plates.

The metals constituting planographic printing plates are usually aluminum or zinc. In operation the plate is successively contacted with a grease or ink spreading medium or inking system, and a water spreading medium or dampening system. The plate has been previously processed by methods known to the lithographer to render predetermined areas ink or grease retentive and other areas water retentive accordin to the picture or legend to be printed.

he water retentive surface of the printing plate during the printing operation, therefore, carries a thin film of moisture which, in the presence. of the oxygen of the air, would cause the wet surface of the plate to corrode or oxidize, with the formation of the oxide, hydroxide or basic carbonate of the metal constituting the plate. The oxide or hydroxide of aluminum and the oxide, hydroxide or basic carbonate of zinc, which are the respective oxidation products of these metals, are grease sensitive, and as they are formed on the water retentive surface of the plate they would cause it to gradually become grease or ink retentive and to scum or tint, destroying the value of the impression. In order to retain the desired areas water retentive and ink repellent during the entire period of the printing operation, certain substances, principally acids, salts or gums or combinations of these materials well known in the lithographic trade, are added to the dampening water to maintain the water retentive film and prevent the formation of oxidation products or to remove them as they are formed. The printing plate is consequently subject to the corrosive action of the above mentioned acids and salts, the use of which cannot be obviated in the present state of the art, and which is detrimental to the printing image.

Therefore the object of this invention is to provide a dampening system for dampening the printing plate of a planographic printing press, without corrosion of the printing plate.

Other objects and certain advantages will be more fully set forth in the description of dampening rollers 3.

the accompanying drawings, forming a part of this specification, in which:

Figure 1 is a diagrammatic end view of the plate cylinder and dampening apparatus of a rotary lithographic press which is used as exemplary for purposes of disclosure in the following description.

As disclosed, the plate 1 is mounted upon the rotatable cylinder 2 of a lithographic press. The ink distributing system is not disclosed and can be of any conventional type. The dampening system comprises two These rolls contact the cylinder of the press to keep the plate dampened. Rotating in peripheral engagement with these rollers is a distributing roller 4. The dampening fluid is placed in a trough 5 in which a fountain roller 6 is partly emersed. Between this fountain roller and the distributing roller 4 is a ductor roller 7, which is shiftable into engagement with the fountain roller and distributing roller alternately (by means not shown) for predetermined periods to control the amount of dampening liquid carried to the printing plate.

The material of which the various rollers are constituted has been chosen in the past in regard to the desired physical properties of each particular roller. For instance, the dampening and ductor rollers have usually consisted of brass stocks or cores covered with successive layers of flannel, with an outside covering of molleton or other cotton fabric. Occasionally, rubber or composition coverings have been used instead of flannel, the outer covering being molleton as described above. usually been constructed of smooth brass tubing. These rollers have usually been journalled. in the same frame as the press cylinder or in an associated frame.

Our research has determined that one cause of printing plate corrosion on a lithographic press has been the unsuspected formation of an electrolytic cell in which the dampening liquid functions as an electrolyte, the press frame work as an external circuit, the printing plate as the anode and the liquid feeding rollers as the cathode. This condition obtains both when a zinc or an aluminum printing plate is used in combination with brass cored dampening or brass dis- The distributing roller has tributing rollers. The result is that the tic action which takes place, caused by the. difl'erence in electro-potential (or solution pressure) between the metals constituting the elements of the cell.

-In the accompanying diagram electrolytic action has been found to take place between the printing plate 1 which constitutes the anodic element of the cell and the brass used in the construction of the two dampening rollers 3, the distributing roller 4 and the ductor roller 7, which constitutes the cathodic element of the cell. This electrolytic action causes accelerated corrosion of the printing plate 1 and protection against corrosion of the rollers 3, 4 and 7. The fountain roller and fountain pan, although usually constructed of brass, do not contribute to this electrolytic action since a complete circuit is never formed through them and the printing plate.

We provide two remedies for this condition:

1. Use materials for the rollers or parts contacted 'by the dampening liquid which have zero electromotive potential in relation to the plate and the liquid.

2. void an external circuit.

According to the first method dampening and distributing rollers are provided which are all of non-metallic composition or of the same composition as the printing plate or some non-metallic and some the same as the plate so that said series of rollers is at the same potential as the late.

According to the secon method, the rollers or plate are electrically insulated from the press frame or from each other to avoid an external circuit.

While the avoidance of the electrolytic cell may have been unconsciously and accidentally accomplished in the past by the choice of certain combinations of material for different reasons still, this electrolytic cell has been equally accidentally formed on most presses in commercial use at the present time and its existence neither suspected nor remedied.

It is also to be noted that this invention applies equally to all planographic printing presses employing metal printing plates and dampening solutions, and particularly to offset, direct rotary and flat-bed lithographic presses.

We wish therefore to be limited only by the following claims:

1. A lithographic press wherein the impression member comprises a metallic plate, dampening rollers, some of which are, in contact with said plate, all of said rollers being at the same electric potential as the plate.

2. In a lithographic press, an impression member comprising a metallic plate and a series of rollers in contact with one another for depositing a dampening liquid on said plate, all of the rollers of said series being at thesame electric potential as the plate.

In witness whereof, we hereunto subscribe our names.

I a ROBERT F. REED.

WALTER SCHOLL. 

